In diesel engines, in order to trap particulate matters in exhaust gas discharged from cylinder and to burn and remove the particulate matters, an oxidation catalyst and a trapping device, which traps the particulate matters in the exhaust gas, are disposed in an exhaust passageway.
In those engines, when the temperature of an exhaust passageway is kept low for many hours as a result of the engine being kept running under low load for many hours, particulate matters accumulate upstream of an upstream end face of the oxidation catalyst, or more specifically, on the upstream end face of the oxidation catalyst and an inner wall of a diametrically expanded pipe which is expanded diametrically from a downstream end of an upstream small-diameter pipe and where the flow velocity of exhaust gas drops. In addition, in raising the temperature of exhaust gas by the oxidation catalyst by supplying unburned fuel into the exhaust passageway to forcibly burn and remove the particulate matters that have accumulated on the trapping device, when the unhurried fuel is not sprayed uniformly or when the temperature of the exhaust passageway is low, the unburned fuel adheres to a portion of the exhaust passageway which is situated further upstream of the upstream end face of the oxidation catalyst. The unburned fuel so accumulating is then subjected to a roasted condition as a result of the engine being run thereafter to grow into particulate matters which accumulate as described above.
In this respect, there has been proposed an apparatus for removing the matter that has accumulated on an oxidation catalyst by raising the temperature of exhaust gas on an upstream side of the oxidation catalyst, as described in JP-A-2004-257267 (PTL 1), for example.
However, since no catalyst exists on the piping laid out upstream of the upstream end face of the oxidation catalyst, if a great amount of particulate matters accumulate on such a place, it is not possible to burn and remove the particulate matters forcibly.
Thus, when a great amount of particulate matters accumulate, the exhaust passageway has to be disassembled and opened to clean and remove the accumulated particulate matters mechanically. However, such a cleaning and removal work involves disassembling and opening the exhaust passageway where the particulate matters accumulate, so that there may be a fear that the particulate matters are scattered.